Modular pressurized coal combustion (mpcc)  for flexible generation

ABSTRACT

A modular pressurized combustion system for flexible energy generation is provided. The system comprises a plurality of pressurized combustion boilers, at least one compressor configured to provide pressurized oxidizer gas to each of the plurality of pressurized combustion boilers in parallel, and at least one feeder configured to provide fuel to each of the plurality of pressurized combustion boilers in parallel. The system further comprises a flue gas input unit configured to provide recycled flue gas to each of the plurality of pressurized combustion boilers in series, at least one pressurized heat recovery unit configured to receive a flue gas output stream from each of the plurality of pressurized combustion boilers, and at least one particle filter configured to filter a flue gas output stream from the pressurized heat recovery unit. The system also comprises an integrated pollutant removal unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional Application Ser. No. 62/878,063 filed on Jul. 24, 2019, which is incorporated herein by reference in its entirety. This application further claims priority from U.S. Provisional Application Ser. No. 62/880,558 filed on Jul. 30, 2019, which is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH AND DEVELOPMENT

This invention was made with government support under RFP-89243319CFE000026, DE-FE0009702, and DE-FE0029087 awarded by the U.S. Department of Energy. The government has certain rights in the invention.

BACKGROUND OF THE DISCLOSURE

The field of the disclosure relates generally to fuel combustion systems. More specifically, the field of the disclosure relates to modular pressurized coal combustion (MPCC) for flexible energy generation.

The rapid addition of intermittent renewable energy (IRE) has created a growing need for responsive, dispatchable generation to provide grid stability. However, existing coal power plants, which have been pressed into operating more flexibly than originally designed in response to the growth of IRE, are being retired at alarming rates due in some cases to not being cost competitive (e.g., in the U.S., where cheaper natural gas power often prevails), environmental concerns, or not being able to be flexible enough, and these retirements are putting the reliability of the grid at risk. Moreover, coal adds to the diversity of the overall generational mix, an important hedge against future changes, e.g., price volatility of natural gas. In view of this, a new breed of coal power plants is needed that is clean, efficient and flexible to meet the changing market demands. Also, due to the impact of CO₂ on climate change, the potential ability to be readily retrofitted a carbon capture plant is also an important feature for future coal power plants.

Due to its large reserves, ease of transportation and storage, low price, coal is expected to persist as one of main energy sources for generating power into the distant future. As more of the grid electricity is generated from intermittent renewable sources (IREs), the existing coal-fired power plants, optimized for baseload, are being increasingly relied on as load-following resources, which adds challenges to plant operations and hurts the economics of the plant. There is a need for a new concept for coal plants, where the plant will have a high efficiency (>40% HHV), increased operational flexibility with high ramp rates and minimal reduction of efficiency at part load, modular construction with low capital cost, and low emissions with the potential to be retrofitted for carbon capture without significant plant modifications. Additional features include integration with energy storage, minimized water consumption, reduced design, construction and commissioning schedules, enhanced maintenance features, integration capability with coal upgrading, and natural gas co-firing capability. The following disclosure is designed to address this need. The systems and methods described herein are intrinsically modular in design, flexible, carbon-capture ready, and have high efficiency and low water use.

BRIEF DESCRIPTION OF THE DISCLOSURE

In one aspect, a modular pressurized combustion system for flexible energy generation is provided. The system comprises a plurality of pressurized combustion boilers, at least one compressor configured to provide pressurized oxidizer gas to each of the plurality of pressurized combustion boilers in parallel, and at least one feeder configured to provide fuel to each of the plurality of pressurized combustion boilers in parallel. The system further comprises a flue gas input unit configured to provide recycled flue gas to each of the plurality of pressurized combustion boilers in series, at least one pressurized heat recovery unit configured to receive a flue gas output stream from each of the plurality of pressurized combustion boilers, and at least one particle filter configured to filter a flue gas output stream from the pressurized heat recovery unit. The system also comprises an integrated pollutant removal unit.

In another aspect, a process for flexible energy generation using a modular pressurized combustion system is provided. The process comprises providing, with at least one compressor, pressurized oxidizer gas to each of a plurality of pressurized combustion boilers in parallel, providing, with at least one feeder, fuel to each of the plurality of pressurized combustion boilers in parallel, and providing, with at least one flue gas input unit, recycled flue gas to each of the plurality of pressurized combustion boilers in series. The process further comprises recovering, with at least one heat recovery unit, heat from a flue gas output stream received from each of the plurality of pressurized combustion boilers, filtering, with at least one particle filter, a flue gas output stream from the at least one pressurized heat recovery unit, and cooling, with at least one integrated pollutant removal unit, a particle-free flue gas output stream received from the at least one particle filter.

In yet another aspect, a system for controlling wall heat flux in a pressurized coal combustion environment is provided. The system comprises at least one burner and at least one low-mixing, axial-flow boiler.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings illustrate various aspects of the disclosure.

FIG. 1 is a simplified process flow diagram illustrating a modular pressurized coal combustion plant with four modular boilers in accordance with one aspect of the present disclosure.

FIG. 2 is a simplified process flow diagram illustrating a staged, pressurized oxy-combustion plant retrofitted from the modular pressurized coal combustion plant shown in FIG. 1 in accordance with one aspect of the present disclosure.

FIG. 3A is a schematic illustration of a boiler that includes a burner with a fuel port positioned between nested inner and outer oxidizer ports in accordance with one aspect of the present disclosure.

FIG. 3B is a schematic illustration of a boiler that includes a burner with an oxidizer port nested within an outer fuel port in accordance with another aspect of the present disclosure.

FIG. 4A is a graph summarizing wall heat flux associated with the combustion of fuel with a particle size range ranging from about 5 μm to about 200 μm, as is used in conventional pressurized combustion (PC) boilers, in a low-mixing, axial-flow boiler in accordance with one aspect of the present disclosure. Negative heat flux is associated with heat transferred from the inside (fire side) of the boiler wall to the outside (water side) of the boiler wall.

FIG. 4B is a graph summarizing wall heat flux associated with the combustion of fuel with a bimodal particle size range ranging from about 10 μm to about 200 μm and from about 1600 μm to about 2000 μm in the boiler of FIG. 4A.

FIG. 5A is a heat map illustrating a temperature contour observed by an operational simulation of the boiler of FIG. 3A.

FIG. 5A is a heat map illustrating a temperature contour observed by an operational simulation of the boiler of FIG. 3B.

FIG. 6A is a graph summarizing the axial distribution of wall heat flux for the boiler of FIG. 3A.

FIG. 6B is a graph summarizing the axial distribution of wall heat flux for the boiler of FIG. 3B.

FIG. 7A is a simplified flow chart illustrating staged combustion system with flue gas recirculation.

FIG. 7B is a graph illustrating a representative profile of flue gas temperature during operation of the staged combustion system of FIG. 7A.

FIG. 8A is a heat map illustrating a temperature profile of a flame from a swirl stabilized burner within a boiler.

FIG. 8B is a heat map illustrating a temperature profile of a flame from an axial jet burner within a boiler.

FIG. 9 is a graph comparing the wall heat flux of boilers outfitted with the swirl stabilized burner of FIG. 8A (A) and the axial jet burner of FIG. 8B.

FIG. 10 is a graph comparing measured and simulated particle temperatures during early stage operation of a boiler in accordance with one aspect of the disclosure.

FIG. 11A is a graph summarizing thermal energy input and flue gas oxygen concentration during operation of a boiler in accordance with one aspect of the disclosure at an energy input of about 50 kW.

FIG. 11B is a graph summarizing thermal energy input and flue gas oxygen concentration during operation of a boiler in accordance with one aspect of the disclosure at an energy input of about 100 kW.

FIG. 11C is a graph summarizing thermal energy input and flue gas oxygen concentration during operation of a boiler in accordance with one aspect of the disclosure at an energy input of about 120 kW.

FIG. 12 is a graph summarizing char gasification and oxidation rates during operation of a boiler in accordance with one aspect of the disclosure at boiler pressures of about 1 bar and about 15 bar.

FIG. 13 is a graph summarizing a furnace wall temperature profile during operation of a boiler in accordance with one aspect of the disclosure at an energy input of about 120 kW.

Those of skill in the art will understand that the drawings, described below, are for illustrative purposes only. The drawings are not intended to limit the scope of the present teachings in any way.

DETAILED DESCRIPTION OF THE DISCLOSURE

In various aspects, a Modular Pressurized Coal Combustion (MPCC) system for flexible power generation is described. The disclosed MPCC system incorporates pressurized coal combustion with a modular boiler design to achieve efficient power generation over a variety of operating conditions.

A high-level process flow diagram for an MPCC system is provided in FIG. 1. FIG. 1 is an exemplary embodiment of a simplified process flow diagram for a modular pressurized coal combustion plant with four modular boilers in accordance with the present disclosure. The MPCC system includes multiple pressurized combustion boilers arranged in parallel. As illustrated in FIG. 1, four pressurized boilers are included, although the number of pressurized boilers may vary depending on the application of the MPCC system. In various aspects, the boilers nominally have the same design and operating conditions as described below.

Referring again to FIG. 1, the MPCC system further includes an air compressor operatively coupled to each of the multiple boilers at an air inlet provided at each boiler. The air compressor is configured to compress air entering each of the multiple boilers before entering the boilers with crushed or pulverized coal from a coal feeder as described below. Any suitable air compressor may be used in the MPCC system without limitation. Non-limiting examples of suitable air compressors include single stage compressors and multi-stage compressors. In various aspects, the pressurized air as delivered to each of the boilers ranges from about 10 bar to about 40 bar (absolute). In other aspects, the air pressure delivered to each of the boilers may be 10 bar, 12 bar, 14 bar, 15 bar, 16 bar, or higher. In the MPCC system shown in FIG. 1, 16 bar (absolute) is used.

In some aspects, the air compressor may be a multi-stage compressor with intercooling. In other aspects, the intercooling heat exchanger may deliver the extracted heat to another element or process of the MPCC system as described below.

Pulverized coal is delivered via a feeder with a small amount of recycled flue gas (BFW) to each of the multiple boilers via a fuel inlet provided at each boiler. In various aspects, the coal and pressurized air are delivered to each of the boilers in roughly equal amounts. In various aspects, the pulverized coal includes a plurality of fuel particles characterized by a particle size distribution. The plurality of fuel particles may have any suitable size distribution without limitation. In one aspect, the plurality of fuel particles may have a continuous particle size distribution ranging from about 5˜2000 μm. In another aspect, the plurality of fuel particles may have a continuous particle size distribution ranging from about 5˜200 μm. In an additional aspect, the plurality of fuel particles may have a bimodal particle size distribution ranging from about 5˜200 μm and from about 1600˜2000 μm. Without being limited to any particular theory, the characteristic fuel particle size distribution may be selected to modulate one or more operating parameters of the MPCC system as described in the examples below. Non-limiting examples of operating parameters that may be modulated through selection of fuel particle size distribution include boiler wall temperature profile and char burnout.

Referring again to FIG. 1, the heat of combustion produced by each boiler is transferred to a power cycle (abbreviated SC in FIG. 1) to generate electricity. Any suitable power cycle may be used by the MPCC system without limitation. Non-limiting examples of suitable power cycles include supercritical (SC) steam-Rankine cycles (as shown in FIG. 1), high-efficiency advanced ultra-supercritical (A-USC) steam-Rankine cycles, indirect-fired supercritical CO₂ cycles, and any another suitable power cycle. The flue gas streams of each of the multiple boilers exits at a flue gas outlet provided at each boiler.

In various aspects, the MPCC system also includes a pressurized heat recovery unit. Any suitable heat recovery unit may be used in the MPCC system without limitation. Downstream of the pressurized boilers, the flue gas streams are combined and fed into the pressurized heat recovery unit (HP Heat Recovery). In this unit, heat is extracted and integrated into the power cycle and the flue gas is cooled to slightly above the acid dew point temperature.

In various additional aspects, the MPCC system also includes a particle filter. Any suitable particle filter may be used in the MPCC system without limitation. After the flue gas is processed by the pressurized heat recovery unit, fly ash particles in the flue gas are removed by a particle filter (PM filter).

In various aspects, the MPCC system further includes a pollutant removal unit. Any suitable pollutant removal unit may be used by the MPCC system without limitation. After particulate removal, the flue gas is further cooled and processed by the pollutant removal unit as illustrated in FIG. 1. In some aspects, the pollutant removal unit is an integrated pollutant removal (IPR) unit. The IPR unit is a single, direct-contact cooling (DCC) column, in which the flue gas flows against a stream of cooler water, thereby reducing the flue gas temperature and resulting in condensation of the flue gas moisture. In some aspects, the water leaving the bottom of the column is at sufficiently high temperature that can be used for boiler feed water (LPFW) heating and/or to supply additional energy to the power cycle (SC) as illustrated in FIG. 1, thereby improving plant thermal efficiency. Due to the high-pressure operation of the MPCC system, sulfur-containing and nitrogen-containing species within the flue gas are dissolved in the cooling water and removed by a neutralizer. This process of pollutant removal, which is effective only under pressure, combined with latent heat recovery, enhances the efficiency of the MPCC process in various aspects. Referring again to FIG. 1, the IPR unit further includes a flue gas outlet that removes treated flue gas from the IPR unit. In some aspects, the clean, particle-free flue gas exiting the IPR unit is heated back to a higher temperature using part of the IPR heat via a heat exchanger.

In various aspects, the MPCC system further includes an expansion turbine to transform the thermal energy of heated flue gas into power. Any suitable expansion turbine may be used in the MPCC system without limitation. Non-limiting examples of suitable expansion turbines include single-stage expansion turbines and multiple-stage expansion turbines. As illustrated in FIG. 1, heated flue gas is delivered to a single- or multi-stage expansion turbine to produce power. In some aspects, if multi-stage compressors and multi-stage expansion turbines are employed, the compression heat will be recovered by intercooling and then this heat will be used to heat the flue gas between turbine stages to increase the power output, as illustrated in FIG. 1. In this way, most of the work used by the compressors is compensated by the power generated from the expansion turbines. The remaining part of the compressor work becomes the auxiliary load of the plant.

In various aspects, the MPCC system has several important advantages over the conventional atmospheric-pressure PC plant. Advantages include:

1) Higher efficiency through recovering flue gas latent heat—As mentioned above, with high pressure, the latent heat from the flue gas can be utilized to increase plant efficiency. The temperature at which moisture condensation occurs in the flue gas is strongly dependent on operating pressure. The significant increase in condensation temperature makes it feasible to utilize the latent heat at pressure. Also, the extra power produced by integrating this latent heat into the power cycle is considerably higher than the net auxiliary load for pressurization, so that a power plant incorporating the proposed concept has a higher plant efficiency. Calculations show that, for the plant configuration shown in FIG. 1, the proposed plant has ˜1.3 percentage points increase in plant efficiency compared with a conventional PC plant utilizing a similar steam cycle.

2) Economical pollutant removal—In a pressurized system, SO_(x) and NO_(x) and some mercury can be removed simultaneously in a cooling column. The advantages of this approach over others include: 1) the capture of SO_(x) and NO_(x) occurs simultaneously, which is more economical than separate removal processes such as selective catalytic reduction (SCR) for NO_(x) removal and sorbent injection for SO₂; 2) large pieces of equipment, like SO_(x) scrubbers and SCRs, are eliminated, resulting in significant capital cost savings; and 3) acid gas condensation is controlled to occur only in a single vessel, eliminating the chances of corrosion in other parts of the system.

3) Reduced gas volume—Compared with atmospheric pressure PC combustion, the overall volume of gas is significantly reduced in a pressurized system. This provides further opportunity to reduce the size of the boiler, pumps, and other equipment. Heat loss to the ambient is also reduced. Importantly, the volume of gas undergoing treatment for removal of ash and other contaminants is reduced, while the concentrations of these contaminants is increased, making their removal easier and more cost effective.

4) Improved coal combustion rate—In coal-fired combustion systems, the amount of air supplied is kept to a minimum to avoid efficiency loss and to minimize the auxiliary load associated with air delivery. In addition, it is important to keep the amount of unburned carbon in the fly ash below levels required for fly ash reuse applications. In a conventional PC plant, the oxygen concentration in the flue gas is normally kept above a minimum value, typically 2.5 vol %. However, studies have shown that coal conversion rates under pressurized conditions are higher, because both char oxidation and gasification rates increase, as demonstrated in the Examples below. Also, the gas volume in a boiler decreases proportionally with pressure, reducing velocity and increasing residence time. This further increases the coal conversion at the exit of the boiler. Therefore, the oxygen concentration in the flue gas can be smaller in a pressurized boiler, effectively reducing the amount of air needed. In addition, with enhanced coal combustion rate, the coal particle size can be larger, which means the auxiliary load for coal pulverizing can be reduced.

5) Increased combustion performance of lower-quality fuels—Some low-rank fuels, such as lignite, have limited use due to their high moisture and low energy content. Since much of the latent heat in the flue gas can be captured in pressurized combustion, the effective heating value of “low-Btu” fuels can be significantly increased.

6) Modular boiler construction—An important advantage of the proposed process is the ability to modularize the construction of the pressurized boiler. Because of the long, thin nature of pressure vessels, they can be built in a factory using skilled labor and high-quality control procedures, and then shipped to the power plant location. This approach is particularly important to the U.S., as some recent advanced coal technology projects have encountered construction delays and cost overruns due to the inability to ensure large numbers of experienced craftsmen to work in remote, rural locations where power plants are often sited. The use of modular construction will facilitate lower construction costs, on-time and within-budget plant construction, and better quality control.

7) Improved plant flexibility—Compared with a conventional PC power plant, the operating flexibility of the proposed plant is increased due to the parallel boiler design. The minimal load for a typical conventional PC plant is ˜25%. There is an efficiency drop at part-load operation, due in part to the mismatch of heat transfer in the radiant and convective sections of the boiler. For the proposed conceptual plant, 25% load can be easily achieved by just shutting down three boiler modules. The efficiency drop caused by heat transfer mismatch can be minimized as the operating condition of the remaining module is full load. Thus, a much deeper turn-down can be achieved with the modular design. Also, the ramp rate and cool/warm start-up time of the proposed conceptual plant should also be higher than a conventional PC plant since the size of each boiler module is relatively small.

8) Carbon-capture ready—The proposed plant can be readily retrofitted to the staged, pressurized, oxy-combustion (SPOC) process, one of the most promising carbon capture technologies for coal power plants. FIG. 2 shows an oxy-coal process retrofitted from the process shown in FIG. 1. As compared to the MPCC system of FIG. 1, the SPOC system of FIG. 2 includes an air separation unit (ASU) added between the compressor stages to produce oxygen that is further compressed and delivered to each of the multiple pressurized boilers. For a typical cryogenic ASU, more than 98% of the energy load is for air compression. As air is already compressed in this process, the energy penalty of adding an ASU is minimal. In addition, the expansion turbines of the MPCC system of FIG. 1 are replaced in the SPOC system of FIG. 2 by a CO₂ purification unit (CPU) to produce CO₂ that is ready for transportation, utilization and/or storage. Since the expansion turbines are removed, the compression heat extracted from the multi-stage compressors is integrated into the power cycle to increase power output. In addition, the multiple boilers of the SPOC system of FIG. 2 are connected into a series-parallel configuration, unique to the SPOC process, in which a small amount of the flue gas coming out of the last-stage boiler (RFG) is recycled back into the first stage boiler. This recycled flue gas is used to dilute the oxygen entering the first-stage boiler. As illustrated in FIG. 2, part of the flue gas coming out of the first-stage boiler is fed into the second stage to dilute the oxygen flow in this stage. The same process occurs for all downstream stages (i.e., oxygen is always mixed with part of the flue gas from the previous stage before it enters the present stage). This unique mode of operation minimizes flue gas recycle and maximizes efficiency. By adjusting the flow rates of the flue gas entering each stage, all stages can have similar operating conditions. Therefore, the plant still maintains high flexibility, since low load can still be achieved by shutting down one or more boilers.

Dry-Feed Pressurized Combustion Boiler Design

Thermal radiation from a particle-laden flue gas stream can be greatly enhanced by pressure. Utilizing conventional coal combustion boiler designs under pressure can lead to excess wall heat fluxes and damages to the water-cooling walls. Therefore, a new boiler design is required for pressurized coal combustion.

A novel method is disclosed herein to control wall heat flux to within an acceptable level under pressurized coal combustion environment. This method incorporates two approaches: creating a low-mixing, axial-flow system and combusting coal particles with a tailored size distribution, to distribute heat release.

Conventional pulverized coal (PC) combustion boilers typically utilize tangential flow to enhance mixing and increase particle residence time, and also utilize very fine coal particles (typical mean and maximum sizes are around 75 μm and 200 μm, respectively) to increase burning rate. All these features are to ensure complete char combustion. In a pressurized combustion boiler, complete char combustion is less of a concern due to the high oxygen partial pressure and longer residence time. Therefore, a low-mixing, axial-flow boiler can be utilized to distribute heat release and thus lower the peak wall heat flux. Unlike a tangentially fired combustion boiler, which releases all the combustion energy in a short distance, a low-mixing, axial-flow boiler can create a longer flame and release combustion energy in a longer distance. Also, a much wider particle size range can be utilized to help distribute heat release. Due to different heating rates, different sized particles ignite at various locations, and burn at different speeds. A wider particle size range can effectively distribute the release of the combustion energy, as illustrated in the Examples below. With a tailored particle size distribution, the heat flux profile along the height of the boiler may be modulated, providing a means of optimizing steam integration.

With above design concepts, different burn configurations can be utilized for enhanced heat flux control in a coal-fired SPOC system. Non-limiting examples of burn configurations are illustrated in FIGS. 3A and 3B. The burner designs can be used independently or coupled with particle size distribution control to enhance the ability to control heat flux to the boiler tubes. This allows additional flexibility to design the boiler tubes for more efficient steam production across a wider variety of operating conditions. FIGS. 3A and 3B shows two examples of burner and boiler designs. The two designs have the same boiler geometry: a cylinder combustor with water-cooled walls, but the burners have different configurations. In the first configuration, the burner is a co-axial flow system, which consists of three streams. The inner stream is the inner oxidizer, the outer stream is the outer oxidizer, and the coal stream is between the two oxidizer streams together with a small amount of carrier CO₂. On both the inner and outer side of the exit of the coal stream tube, there is a flame stabilizing anchor. In the second configuration, there are only two streams, all the oxidizer is fed in to the center tube, coal and carrier CO₂ is surrounding the oxidizer. Both burner configurations can create a relatively long flame, compared with the tangentially-fired boilers typically used in conventional coal combustion boilers. The unique burner designs and use of particle distributions effectively control the burn, provide additional heat distribution control, and provide an improved alternative option to conventional SPOC process burners/boilers.

EXAMPLES

The following Examples describe or illustrate various embodiments of the present disclosure. Other embodiments within the scope of the appended claims will be apparent to a skilled artisan considering the specification or practice of the disclosure as described herein. It is intended that the specification, together with the Examples, be considered exemplary only, with the scope and spirit of the disclosure being indicated by the claims, which follow the Examples.

Example 1: Projected Cost and Performance Estimates

To evaluate the cost and performance of a modular pressurized coal combustion power plant as described above, the following experiments were conducted. A preliminary process analysis was carried out using plant configurations and steam cycles parameters as summarized in Table 1 below. NETL Base Case was selected as representative of a conventional supercritical (SC) steam-Rankine cycle pulverized coal (PC) plant. MPCC Case 1 employed a modular pressurized coal combustion (MPCC) system as described above and a SC steam cycle with single reheat. MPCC Case 2 employed an MPCC system and an advanced ultra-supercritical (A-USC) steam-Rankine cycle with double reheat. The estimated performance for each of these cases is summarized in Table 1.

TABLE 1 Performance comparison for conceptual plant with different steam cycles. Net efficiency, Case Steam pressure/temperature/reheat temp HHV (%) Conventional SC 3500 psig/1100° F./1100° F./— 40.7 Power plant MPCC Case 1 3500 psig/1100° F./1100° F./— 42.0 MPCC Case 2 4200 psig/1300° F./1200° F./1200° F. 44.3

The levelized cost of electricity (LCOE) for the MPCC Case 1 was expected to be less than that for a conventional PC plant with the same power cycle. Even though the air compressors and flue gas expansion turbines added capital cost to the plant, the integrated pollutant removal (IPR) unit of MPCC Case 1, which combined latent heat recovery with SO_(x) and NO_(x) removal in a compact direct-contact cooling (DCC) column, replaced the traditional and expensive emission control equipment of the Conventional SC Power Plant. In addition, in pressurized combustion power systems such as MPCC Cases 1 and 2, the boilers, pumps, and other equipment were smaller, and though the pressure vessels for the boilers added additional cost, the modular boiler design allowed mass production of boilers in a factory using skilled labor with high-quality control procedures, which reduced estimated construction costs. Further, for a given-sized plant (i.e., electricity output), the higher efficiency of the MPCC Cases 1 and 2 lead to lower capital and operational costs as compared to the Conventional SC Power Plant. Considering previous economic analyses conducted for SPOC process (not included), the LCOE for MPCC plants is expected to be ˜20% less than a conventional PC plant of comparable size and power cycle configuration.

Example 2: Effect of Fuel Particle Size Distribution on Performance of Axial Flow Boiler

To evaluate the performance of a boiler with a low-mixing, axial-flow burner as described above, the following experiments were conducted.

To assess the effects of fuel particle size on wall heat flux, a simulation of combustion within a boiler with a low-mixing, axial-flow burner under oxy-combustion conditions was conducted for fuel characterized by two different ranges of particle sizes: a continuous particle size range (10-200 μm) that was representative of the fuel used in conventional PC boilers, and a bimodal particle size range (10˜200 μm and 1600˜2000 μm). Although the boiler design simulated in these experiments was capable of performing combustion in both air combustion mode and oxy-combustion modes, oxy-combustion typically exhibited higher radiative heat flux than air combustion due to higher CO₂ concentration. Therefore, oxy-combustion was used for all cases in the experiments of the present example to illustrate the effectiveness of the combustion method using the low-mixing, axial-flow burner in controlling wall heat flux.

FIGS. 4A and 4B summarize the simulated heat flux as a function of axial distance within the boiler. Note that negative heat flux denotes heat transfer from the inside (fire side) of the wall to the outside (water side) of the wall. FIG. 4A summarizes heat transfer estimated for combustion of fuel with the continuous particle size range (5˜200 μm) and FIG. 4B summarizes heat transfer estimated for combustion of fuel with a bimodal particle size range (10˜200 μm and 1600˜2000 μm). As illustrated in FIG. 4A, combustion of the fuel particles with a continuous particle size distribution resulted in a relatively high and narrow peak in surface heat flux. By contrast, combustion of the fuel particles with a bimodal particle size distribution resulted in a lower magnitude, but broader distribution of surface heat flux, as illustrated in FIG. 4B.

The temperature contours and wall heat fluxes of two burner configurations, shown illustrated in FIG. 3A and FIG. 3B, were also evaluated by simulating combustion of fuel particles with a bimodal particle size range (5˜200 μm and 1600˜2000 μm) at an operating pressure of 15 bar. FIGS. 5A and 5B illustrate the temperature contours within the boilers with burner designs as illustrated in FIGS. 3A and 3B, respectively. FIGS. 6A and 6B are graphs summarizing the estimated wall heat flux for the boilers with burner designs as illustrated in FIGS. 3A and 3B, respectively.

FIGS. 8A and 8B are thermal maps illustrating the temperature within boilers fitted with a swirl-stabilized burner and an axial jet burner, respectively. FIG. 9 is a graph comparing the axial distribution of wall heat flux corresponding to the thermal map shown in FIG. 8A (curve marked A) and corresponding to the thermal map shown in FIG. 8B (curve marked B). As illustrated in FIG. 9, the peak heat flux of the swirl-stabilized burner was higher than 800 kW/m² when operating at a pressure of about 15 bar, whereas the peak heat flux of the axial jet burner was less than 500 kW/m². FIGS. 6A and 6B show that incorporation of the burners illustrated in FIGS. 3A and 3B limited peak wall heat flux to lower than 450 kW/m², which is an acceptable level for boiler tube materials.

Example 3: Effect of Combustion Pressure on Char Burnout

To evaluate the effect of combustion pressure on char burnout, the following experiments were conducted.

A simulation of fuel particle combustion within a flue gas composition of 3 vol % of O₂, 6 vol % of H₂O and 91 vol % of CO₂. Combustion reaction kinetics were modeled as simplified 1^(st) order reactions following Smith's approach:

2C(s)+O₂→2CO  (I)

C(s)+H₂O→CO+H₂  (II)

C(s)+CO₂→2CO  (III)

FIG. 12 is a graph summarizing the char reaction rates of oxidation (reaction I), gasification (reactions II+III) and total (reactions I+II+III) at combustion pressures of 1 bar and 15 bar. As illustrated in FIG. 12, oxidation reaction rates increase modestly as combustion pressure increases from 1 bar to 15 bar, but gasification reaction rates increased dramatically for the same combustion pressure increase.

The results of these experiments demonstrated that char reaction rates for oxidation reactions were relatively insensitive to changes in combustion pressure, and that char reaction rates for gasification reactions strongly increased in response to increases in combustion pressure.

Example 4: Performance of Staged, Pressurized Oxy-Combustion (SPOC) Process

To evaluate the effect of combustion pressure on char burnout, the following experiments were conducted.

To evaluate the early-stage temperature history of fuel particles in large-scale boilers of SPOC similar to the boilers described above, a simulation of the SPOC process was conducted. To validate the simulation, experimentally-measured characteristics of boiler firing were also obtained.

FIG. 10 is a graph comparing simulated and experimentally-measured particle temperatures as a function of residence time within an SPOC boiler. Good agreement was demonstrated between the simulated and experimentally observed particle temperature profiles shown in FIG. 10, and qualitative agreement was demonstrated for corresponding flame shapes and flow fields (data not shown). A stable coal flame was achieved in a co-axial flow without a heated wall, which remained stable under firing rate of less than about 8%.

Additional experimental measurements were obtained while operating the SPOC boiler with thermal inputs of 50 kW, 100 kW, and 120 kW of energy. The operating conditions for each thermal input are summarized in Table 2 below.

TABLE 2 SPOC Experimental Operating Conditions. Parameter Thermal input ~50 kW ~100 kW ~120 kW Pressure 15 bara 15 bara 15 bara Oxygen concentration in 35% 35% 32% oxidizer Stoichiometric ratio 1.08 1.12 1.07 Residence time ~10 s ~5 s ~4.2 s O₂ concentration in flue gas 1.9~2.3%   2.5~3.5%   0.7~1.2%   Burnout >99.5%   >99.5%   99.9%  

Additional experimental measurements were obtained for thermal inputs of 50 kW, 100 kW, and 120 kW of energy. The operating conditions for each thermal input are summarized in Table 2 below.

FIGS. 11A, 11B, and 11C are graphs summarizing representative time profiles of thermal input and oxygen concentration within the flue gas of the SPOC boiler operating with thermal inputs of 50 kW, 100 kW, and 120 kW, respectively. Essentially complete fuel combustion was be achieved even with 1 vol % oxygen in the flue gas, due to enhanced gasification reactions under pressurized combustion at 15 bar.

The results of these experiments demonstrated efficient operation of the SPOC boiler at a range of thermal energy inputs at flue gas oxygen concentrations as low as about 1%. 

What is claimed is:
 1. A modular pressurized combustion system for flexible energy generation, the system comprising: a plurality of pressurized combustion boilers; at least one compressor configured to provide pressurized oxidizer gas to each of the plurality of pressurized combustion boilers in parallel; at least one feeder configured to provide fuel to each of the plurality of pressurized combustion boilers in parallel; a flue gas input unit configured to provide recycled flue gas to each of the plurality of pressurized combustion boilers in series; at least one pressurized heat recovery unit configured to receive a flue gas output stream from each of the plurality of pressurized combustion boilers; at least one particle filter configured to filter a flue gas output stream from the pressurized heat recovery unit; and an integrated pollutant removal unit.
 2. The modular pressurized combustion system of claim 1, wherein the integrated pollutant removal unit is a direct contact cooling column.
 3. The modular pressurized combustion system of claim 1, further comprising at least one expansion turbine.
 4. The modular pressurized combustion system of claim 1, further comprising at least one CO₂ purification unit.
 5. The modular pressurized combustion system of claim 1, wherein the pressurized oxidizer gas is air.
 6. The modular pressurized combustion system of claim 1, wherein the pressurized oxidizer gas is oxygen and the at least one compressor further comprises at least one air separation unit.
 7. The modular boiler system of claim 1, wherein the fuel is coal.
 8. The modular boiler system of claim 1, wherein the fuel is a low-rank fuel.
 9. A process for flexible energy generation using a modular pressurized combustion system, the process comprising: providing, with at least one compressor, pressurized oxidizer gas to each of a plurality of pressurized combustion boilers in parallel; providing, with at least one feeder, fuel to each of the plurality of pressurized combustion boilers in parallel; providing, with at least one flue gas input unit, recycled flue gas to each of the plurality of pressurized combustion boilers in series; recovering, with at least one heat recovery unit, heat from a flue gas output stream received from each of the plurality of pressurized combustion boilers; filtering, with at least one particle filter, a flue gas output stream from the at least one pressurized heat recovery unit; and cooling, with at least one integrated pollutant removal unit, a particle-free flue gas output stream received from the at least one particle filter.
 10. The process of claim 9, wherein cooling with at least one integrated pollutant removal (IPR) unit comprises cooling with at least one direct contact cooling column.
 11. The process of claim 9, further comprising: heating, with heat from the at least one IPR unit, a clean particle-free flue gas output stream from the IPR unit, and expanding, with at least one expansion turbine, a heated clean particle-free flue gas output stream received from the IPR unit.
 12. The process of claim 9, further comprising purifying, with at least one CO₂ purification unit, a clean particle-free flue gas output stream received from the IPR unit.
 13. The process of claim 9, wherein providing pressurized oxidizer gas comprises providing pressurized air.
 14. The process of claim 9, wherein providing pressurized oxidizer gas comprises providing pressurized oxygen, and the at least one compressor further comprises at least one air separation unit.
 15. The process of claim 9, wherein providing fuel comprises providing coal.
 16. The process of claim 9, wherein providing fuel comprises providing a low-rank fuel.
 17. A system for controlling wall heat flux in a pressurized coal combustion environment, the system comprising: at least one burner; and at least one low-mixing, axial-flow boiler.
 18. The system of claim 17, wherein the at least one burner is configured with a co-axial flow of three concentric streams comprised of an inner oxidizer stream, an outer oxidizer stream, and a fuel stream between the inner and outer oxidizer streams.
 19. The system of claim 17, wherein the at least one burner is configured with a co-axial flow of two concentric streams comprised of an inner oxidizer stream and an outer fuel stream.
 20. The system of claim 17, wherein the at least one low-mixing, axial-flow boiler has a cylinder geometry combustor with water-cooled walls. 